Iodide transport is a specialized function of thyroid cells that is critical for thyroid hormone formation. Previous studies in this laboratory focused on iodide transport and iodine metabolism in thyroid. The studies were extended further to investigate other ion-transport processes in thyroid including those involved in pH regulation and those required for epithelial cell metabolism. The transport proteins that were identified in thyroid included an anion-exchange protein (AE2) and a new member of the proton-dependent monocarboxylate transporter family (MCT). These proteins were identified in cultured thyroid cells and thyroid tissue from rat using specific antibodies. cDNA's encoding these proteins were also isolated and characterized. The function of these transport proteins was studied using plasma membrane vesicles and transfected cells. To characterize lactic acid transport activity, it was necessary to develop a new type of heterologous expression system. Unlike reported studies of lactic acid transport, this model system has no endogenous lactic acid transport activity and measures only activity of the cloned transport protein. This model system will also be useful to study other transport proteins from thyroid.